Acoustic apparatus



Nov. 18, 1941. w, BURCK 2,263,233

V ACOUSTIC APPARATUS Filed June 24, 1939 RECTIFIER mo GAIN-INTENSIFYINCDEVICE GAlN-REGULATED AMPLIFIER 4 1 .5 6 AMPLIFIER I 2/ a )AMPLIFIERS IiNVENTOR. WERNER BKRCK 7k Mw L.

ATTORNEY.

Patented Nov. 18, 1941 ACOUSTIC APPARATUS Werner Biirck, Berlin,Germany, asslgnor to Telefunken Gesellschait fiir Drahtlose Telegraphicm. b. 11., Berlin, Germany, a corporation of Germany Application June24, In

6Cialms.

This invention relates to an arrangement adapted to preclude acousticfeedbacks in acoustic apparatus- 1939, Serial No. 280,941

June 24, 1938 In a sound pick-up and acoustic studio, the

by providing a dynamic or sound-volume regulator means such that, whenfeedback occurs, the gain is cut down below the feedback level. However,this scheme has fallen short of giving full satisfaction. For, if thesteady or normal amplification or gain of the installation, that is tosay, the gain in the absence of voice action upon the microphone, is setto a level above the feedback limit, the amplification adjusts itself bythe incipient feedback. The installation stays in a state of feedbackwith moderate volume. If. on the other hand, arrangements are made sothat the gain is raised-only when there is acoustic action upon themicrophone, then. to be sure, the installation will be free fromfeedback-in the absence of sound action on .the microphone: but, afeedback will arise upon the first loud action of sound on themicrophone.

Now, the invention is concerned with an ar- I rangement comprisingvolume-range regulation circuits which is capable of operating with highamplification, while feedbacks are effectively precluded in spite of theuse of circuits of the said kind.

The desired end is obtained according to the invention in the followingway; the amplification of the equipment is no longer regulated only as afunction of the sound intensity acting at the microphone, but inaddition it is regulated in dependence upon the direction of incidenceof the sound upon the microphone. This insures the important advantagethat arrangement of the circuits may be so made that, for instance,waves reflected in the room, or standing waves which are primarilyresponsible for the initiation of acoustic feedback, will cause areduction of the gain of the equipment, whereas the useful or signalsound which impinges upon the microphone mostly irom a definitedirection or within a comparatively narrow solid angle, is utilizedior'an increase in the gain. In other words, the equipment operates inthe following fashion: When the microphone is not acted upon by sigmlsound, the amplification is low. There is no risk of any feedback beingproduced. As the microphon is acted upon by signal sound, the gain israised as a function of the incident signal sound waves. As soon as thereflected sound occurring slightly later assumes such an intensity thatthere is a real risk of an acoustic feedback being produced, the gainis'immediately cut down so that the initiation of, a feedback issuppressed. The time-constants of regulation in this connection must beso chosen that the regulator action will set in within the transit timeperiod of refiected waves in the room or space.

The appended drawing shows an exemplified embodiment of the basic ideaof the invention as applied to an arrangement in which the danger ofacoustic feedback in electro-acoustic transducers is largely diminished.The main microphone, say, for the speaker, indicated at l, is adirectional microphone 2 with a highly concentrated characteristic whichis also shown in the drawing at 1. The electrical oscillationscorresponding to the sound waves which strike the microphone 2 are fedthrough the input amplifier l, the regulated amplifier 5 and the poweramplifier t, and thence to the loudspeaker I. A rectifier and gainintensifying device 8 is subiect to control by output energy from theamplitier 4 and operates to increase the gain in the amplifier t indirect proportion to the amplitude of input energy applied thereto.Adjacent the main microphone or pick-up microphone 2 is mounted theregulator microphone 9. The characteristic I! of this microphone isreniform, as shown in the drawing. This microphone does not pick up inany appreciable degree the direct sound waves which issue from theoriginal sound source to be amplified, such as a speaker positioned atI. In fact, it responds only to such stray sounds as are due toreverberation in the room. 'Ioprevent such stray sound waves from actingalso upon the main or pickup microphone, a covering screen or bafile His.mounted in the rear of the speaker I. Such sound waves as impingeupon the regulator microphone 9 are passed through the input amplifierI! to the rectifier and gain reducing device II and are thence passed tothe joint regulating amplifier I in such a sense that the gain is cutdown.

The amplifier 5 will be understood from the above description to besubject to positive gain regulation by the device l and simultaneouslysubject to negative gain regulation by the device it. In the operationof my invention. one of its characteristics is that as soon as wavespicked up by microphone 9 happen to surpass a certain limiting value,the gain of the equipment is reduced.

What is claimed is:

1. A system to prevent acoustic feedback of electro-acoustic transducersin studio output circuits comprising a studio, a first microphone havingdirectional response characteristics, a second microphone havingreniform response characteristics, and-a screen member, said firstmicrophone located in said 'studio near the source where the soundorginates, said screen member located between the point where the soundoriginates and a wall of said studio, said second microphone located insaid studio so that it will b acted upon by the stray sound waves andsubstantially unimpressed by the original sound, means to combine theoutputs of said microphones so that the gain of the transducer isautomatically controlled as a function of the drection of incidence ofthe sound originating in said studio.

2. A system to prevent acoustic feedback of electro-acoustic transducersin studio output circuits comprising a studio, a first microphone havingdirectional response characteristics, a second microphone havingreniform response characteristics, said first and second microphonesbeing mounted within said studio so that the directions of maximumresponse are opposed, and means to combine the ouputs of saidmicrophones so that the gain of the transducer is automaticallycontrolled as a-function of the direction of the incidence of the soundoriginating in said sound studio.

3. A system to prevent acoustic feedback of electro-acoustic transducersin studio output circuits comprising a studio, a first microphone havingdirectional respons characteristics, a second microphone having reniformresponse characteristics, said first microphone located in said studionear the source where said sound originates, said second microphonelocated in said studio so that it will be acted upon by stray soundwaves and substantially unimpressed by the original sound, a controlledamplifier in the output circuit of said transducer, means connected tosaid first and secondrmicrophone circuits so that the sound oscillationspicked up by the first microphone serve to raise the gain in saidcontrolled amplifier while the stray sound waves picked up by the secondmicrophone serve to cut down the gain of said controlled amplifier.

4. A system to prevent acoustic feedback of electro-acoustic transducersin studio output circuits comprising a studio, a first microphone havingdirectional response characteristics, a second microphone havingreniform response characteristics, said first microphone located in saidstudio near the source where the sound originates, said secondmicrophone located in said studio so that it will be acted upon by straysound waves and substantially unimpressed by the original sound, a firstand a second rectifier, said first microphone connected to said firstrectifier, said second microphone connected to said second rectifier, acontrolled amplifier in the output circuit of said transducer, meansconnected to said first and second microphone circuits so that the soundoscillations picked up by the first microphone by means of said firstrectifier serve to raise the gain in said controlled amplifier while thestray sound waves picked up by the second microphone by means of saidsecond rectifier serve to cut down the gain of said controlledamplifier.

5. A system to prevent acoustic feedback of an -electro-acoustictransducer comprising a sharply directional microphone situated withinthe range of and facing a source of sound waves, a' broadly directionalmicrophone positioned in a back-toback relation to the first saidmicrophone, and means to opposingly combine the outputs of saidmicrophones so that gain of the transducer is automatically reduced inproportion to the input energy derived from the second said microphone.

6. A system to prevent acoustic feedback of an electro-acoustictransducer comprising a sharply directional microphone situated withinthe range of and facing a source of sound waves, 'a broadly directionalmicrophone positioned in a back-toback relation to the first saidmicrophone, means to increase the gain of said transducer in directproportion to the amplitude of energy delivered by the first saidmicrophone, and means to reduce the gain in said transducer indirectproportion to the amplitude of energy derived from the second saidmicrophone.

WERNER BiiRcK.

